Great can be used to refer to both high volume and high quality. Here are two of its definitions:

1. a : notably large in size : huge

b : of a kind characterized by relative largeness —used in plant and animal names

c : elaborate, ample: great detail

2. markedly superior in character or quality

Thus, you could say Our oceans provide a greater quantity and quality of sustenance.

Great can be used to refer to both high volume and high quality. Here are two of its definitions:

1. a : notably large in size : huge

b : of a kind characterized by relative largeness —used in plant and animal names

c : elaborate, ample: great detail

2. markedly superior in character or quality

(Merriam-Webster)

Thus, you could say Our oceans provide a greater quantity and quality of sustenance.